The photochromic glasses are now well known and are characterized by their ability to darken when exposed to actinic radiation, essentially ultraviolet radiation, and to brighten when this excitatory source disappears. Since the invention of these glasses (U.S. Pat. No. 3,208,860), now some 30 years ago, they have been applied with variations in a great number of versions, depending on whether one or the other of the attributes of photochromism was optimized for particular application. In general, the critical attributes of the photochromic glasses for ophthalmic applications are: their color and level of transmission in the clear state (in the absence of actinic radiation), their color (usually gray or brown) and transmission after darkening resulting from exposure to actinic radiation, the low amplitude of the variation in the level of transmission in the darkened state as a function of the temperature, normally between 0 and 40.degree. C., and their ability to brighten reversibly when the excitatory light source disappears.
For example, U.S. Pat. No. 4,746,633 describes glasses for ophthalmic use, having a transmission in the clear state of more than 88% and a transmission of 35-60% after 15 min of exposure to an actinic radiation source, and which then brighten to a minimum transmission level of 75% when this light source is eliminated. These glasses are also characterized by a transmission of less than 63% when the exposure to light occurs at a temperature of 40.degree. C. From numerous examples given in this invention, it appears that the thermal dependence of the transmission in the darkened state increases very significantly when the transmission in the darkened state at 25.degree. C. is decreased. Typically, a difference is much larger than 18.5 points of transmission exists between 25 and 40.degree. C., whereas the transmission in the darkened state at 25.degree. C. is less than 30%, measured at a thickness of 2 mm.
It would be useful to have available a photochromic glass for ophthalmic use, with a refractive index of approximately 1.5, presenting the following properties:
(a) a high transmission T.sub.0 in the clear state, typically more than 86%, preferably more than 88%, at a thickness of 2 mm, and essentially associated with the absence of coloration; PA1 (b) a transmission T.sub.d15(25.degree. C.) in the darkened state, after 15 min of exposure at 25.degree. C. to a sunlight-simulation device, of less than 35% at a thickness of 2 mm; PA1 (c) a transmission T.sub.d15(40.degree. C.) in the darkened state, after 15 min of exposure at 40.degree. C. to a sunlight-simulation device, of less than 45% at a thickness of 2 mm; PA1 (d) a transmission of more than 75% after 1 h of illumination in the absence of actinic radiation; and PA1 (e) a stability of the photochromic properties with respect to the thermal treatment used in the antiglare treatments, such that the difference between the values T.sub.d15(25.degree. C.) of the initial value and that of the glass after thermal treatment of 1 h at 280.degree. C. does not exceed 5 transmission points, in absolute value.
Such a photochromic glass does not exist today, to the knowledge of the applicant.